Breeding new vegetable varieties is based on the presumption of the ability of developing new traits or improving existing traits. Traditionally important characteristics for any crop are disease resistances, shelf life, fruit quality, plant vigour, and yield.
Yield increase is among the most complex quantitative traits to acquire. Many processes and interactions with other aspects of plant and fruit development exist, both genetically and environmentally. For example in cucumber, several studies have been done to analyse the various components that may or might contribute to yield. An extra complicating factor in cucumber is the observation that the genetic base of the presently cultivated cucumbers is rather narrow and offers limited room for recombination and improvement (Meglic et al., Genet. Res. Crop Evol. 46:533-546, 1996). This is especially the case for long cucumbers. They may freely be combined with their shorter relatives such as slicers, Beit Alpha types, or picklings, but the genes that may be introduced from those often have a negative effect on plant vigour or on the long cucumber size, and consequently on their high yield.
Yield that may be attained in greenhouse-grown long cucumbers is very high, with around 80 kg/m2 as an average in the Netherlands in 2008. Yields for the shorter cucumbers are extremely lower, due to a different type of growing and a shorter growing period, but also due to the genetic potential of the crop. In the US, the average yield for cucumbers (mainly slicers combined with gherkins) in 2008 was around 1.6 kg/m2. This indicates the large gap in potential between different growing practices and various types belonging to the same Cucumis sativus species.
Although yield is strongly influenced by environmental factors, studies have shown that e.g. number of female nodes and days to anthesis are negatively correlated with yield per plant (López-Sesé and Staub, J. Amer. Soc. Hort. Sci. 127(6): 931-937, 2002), while e.g. larger leaf size and main stem length are positively correlated with yield per plant (fruit mass) (Serquen et al, J. Amer. Soc. Hort. Sci. 122(4): 522-528, 1997).
One approach that could be used in cucumber growing to increase the yield per m2 is to plant at a higher density. Increasing the plant density however will need a higher number of plants per area, which means higher costs per area. If a plant as such does not produce a higher yield, there has to be another way of recovering those costs. In addition, closer planting may result in a decrease in light intensity which gives poorer plant and fruit development. The higher number of plants per area often also results in a higher disease incidence because of a change in micro-climate. Plants that are suitable for closer planting, for example plants with smaller leaves, are known to have negative side-effects such as slow growth, poorer fruit quality, or lower yield per plant. In addition, this still means higher costs per area for planting material.
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